Devices used for sensing electrical activity in tissue have many uses in modern society. In particular modern electroencephalograms (EEGs) are used for measuring electrical activity in the brains of people for anesthesia monitoring, attention deficit disorder treatment, epilepsy prediction, and sleep monitoring, among other uses. Unfortunately, the complexity and cost of prior modern EEGs typically limits their use to clinics or other facilities where the device can be used on numerous people under the expert attention of a trained medical professional. Using the EEG on numerous people in a clinical setting helps to distribute the cost of the machine to the people which use it. EEGs can cost several thousand dollars.
Trained personnel are used for setting up and operating EEGs because of the complexities involved. Setting up prior EEGs involves preparing the skin of the person for connection of electrodes. The skin is typically prepared by shaving the hair from the area, sanding the skin to remove the outer surface and applying a conductive gel or liquid to the skin before attaching the electrode to the skin. Such extensive skin preparation is needed because contact resistance between the electrode and the skin must be reduced in order for prior EEGs to work properly. Contact resistance in these prior EEGs typically needs to be 20 k ohms or less.
Typical prior EEGs are subject to errors caused by electrical and magnetic noise from the environment surrounding the person. Errors are also caused by slight variations in internal components of the EEG and other sources, such as movement of the person during the operation of the EEG. Environmental noise can be caused by 60 Hz power in electrical wiring and lights in the area where the EEG is used, and other sources. Even the friction of any object moving through the air can cause noise from static electricity. Most or all prior EEGs have two electrodes are connected to the person's head and wires which are run from each of the electrodes to the EEG machine. The routing of the wires and the positions of the noise causing elements in the environment can cause significant errors in the measurements done by the EEG.
Measuring the electrical activity in the brain is difficult because the electrical signal being measured is many times smaller than the noise in the system. In many instances, the noise is on the order of a few volts or a few tens of volts while the electrical signal being measured is only in the microvolt range. This gives a signal-to-noise ratio of 10 6.
Prior EEGs have used very precise differential amplifiers, such as instrumentation amplifiers, to measure the electrical signal. The amplifier is referenced to a common reference such as the leg of the user. Each of the two wires from the two electrodes on the person's head are connected to the inputs of the differential amplifier. The output of the differential amplifier is a voltage relative to the reference which is proportional to the difference in voltage between the two electrodes times a constant. The measurement in this case is very sensitive because the differential amplifier is finding a small difference, the brain signal, between two signals which are 10{circumflex over ( )}6 times as large. These are reasons why small variations in components, the routing of the wires and other factors cause significant errors in the measurement and why prior EEGs are expensive and hard to use.
Another problem with the prior EEGs is that the 60 Hz noise is amplified at the first stage which saturates the signals before they are subtracted. In prior EEGs, designers go to great lengths to design systems that balance or shield the noise to avoid saturation. Systems which use the principle of subtracting two large numbers in measuring a small number are prone to these kinds of problems.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon reading of the specification and a study of the drawings.